This invention relates to a process for the preparation of iminodisuccinic acid alkali metal salts by reaction of maleic acid and ammonia in an aqueous medium in the presence of alkali metal hydroxides and working up thereof. The resulting products can be employed as complexing agents for alkaline earth metal and heavy metal ions in the fields of detergents and cleaning compositions, pharmaceuticals, cosmetics, agriculture, electroplating, building materials, textiles and paper. In these fields, use as a water softener, bleaching agent stabilizer, trace nutrient fertilizer and setting retarder is to be emphasized in particular. The invention furthermore relates to the use of iminodisuccinic acid alkali metal salts in papermaking.
Complexing agents have been employed in large amounts for years. Many complexing agents customary to date, such as ethylenediaminetetracetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), nitrilotriacetic acid (NTA) and various phosphonates, are not biodegradable or are biodegradable to only a limited degree, remobilize heavy metals in surface waters and can even enter drinking water treatment, since they are adsorbed neither in sewage sludges nor in soils. Phosphates are complexing agents which contribute towards eutrophication of surface waters. Summarizing, these are ecotoxicological properties which are found to be a disadvantage nowadays.
It is therefore an important object to develop complexing agents which do not have the ecotoxicological disadvantages to date. Iminodisuccinic acid is thus a complexing agent which shows a ready biodegradability and therefore has an ecotoxicological advantage over the complexing agents to date.
In the future, however, not only will the product properties of chemicals which chiefly enter the environment after use be examined critically under the aspects described, but also the preparation processes. It was thus surprising that an environmentally relevant preparation process could also be found for a chemical which is currently not yet available industrially and has environmentally relevant properties.
For iminodisuccinic acid, the following preparation possibilities based on maleic anhydride or maleic acid and ammonia are known to date: GB 1 306 331 describes the preparation of iminodisuccinic acid from maleic acid and ammonia in a molar ratio of 2:3 to 2:5 at temperatures of 60 to 155.degree. C. For working up, either hydrochloric acid or sodium hydroxide solution are added. In SU 0 639 863, iminodisuccinic acid is prepared from maleic acid and ammonia at a molar ratio of 2:0.8 to 2:1 and temperatures of 110 to 130.degree. C. in the presence of alkali metal hydroxides. JP 6/329 606 describes a three-stage process for the preparation of iminodisuccinic acid. A maleic acid derivative is first reacted with ammonia in an aqueous medium. Alkali metal or alkaline earth metal hydroxides are then added. In the third process stage, a so-called maturation process follows. JP 6/329 607 also describes a three-stage process for the preparation of iminodisuccinic acid. In the first stage, a maleic acid derivative is again first reacted with ammonia in an aqueous medium. Alkali metal or alkaline earth metal hydroxides are then added in the second stage. In the third stage, after addition of a further maleic acid derivative, the reaction is continued. It is expressly stated in this patent application that maleic anhydride, maleic acid or maleic acid ammonium salt are employed as maleic acid derivatives. The desired reaction is said to take place hardly at all with metal salts of maleic acid, so that the aim cannot be achieved.
It is all the more astonishing that, according to the invention, maleic acid and ammonia can be reacted in an advantageous manner to give iminodisuccinic acid in high yields precisely in the presence of alkali metal hydroxides.